In applications in which the pH of a test solution is to be measured potentiometrically, one of the earliest and still widely used types of electrode systems is based on the use of a glass electrode. This type of electrode system usually includes a pH sensitive glass member that is filled with a suitable electrolyte solution, such as potassium chloride in water, in which is immersed a suitable internal reference, such as a silver wire that has been coated with a layer of silver chloride. This type of electrode system also includes a reference electrode comprising a closed body member that is filled with an electrolyte solution, such as potassium chloride in water, in which is immersed an internal reference similar to that used in the pH electrode. Unlike the pH electrode, however, the reference electrode also includes a leak structure or liquid junction for establishing ionic continuity between its internal electrolyte and the test solution. One representative glass pH electrode structure is shown in U.S. Pat. No. 2,256,733, issued in the name of Cary et al. on Sept. 23, 1941. One representative reference electrode structure, which is based on the use of a glass body member, is shown in U.S. Pat. No. 2,925,370, issued in the name of Rohrer on Feb. 16, 1960.
One problem with glass electrodes of both types is the fact that the properties of the internal electrolyte solution can change with time. In pH electrodes, for example, slow evaporation, and/or condensation, the growth of microbes and other influences can result in changes in the activity of the electrolyte. In reference electrodes, on the other hand, diffusion through the liquid junction gives rise not only to changes in the concentration of the electrolyte, but also to cross-contamination of the electrolyte and test solutions.
In dealing with the above problems, various pH electrodes have been designed to operate with solid electrolytes. One example of a pH electrode with a solid electrolyte is shown in U.S. Pat. No. 3,853,731, issued in the name of Gray et al. on Dec. 10, 1974. Another pH electrode of this type is shown in U.S. Pat. No. 4,062,750, issued in the name of Butler on Dec. 13, 1977. A more recent pH electrode of this type is described in my copending application Ser. No. 163,112 filed June 26, 1980. Except in the latter instance, however, these solid electrolyte systems comprise non-uniform structures, i.e., structures which are deposited in a succession of layers.
Attempts have also been made to design reference electrodes having solid electrolytes. One reference electrode of this type is shown in U.S. Pat. No. 4,280,889, issued in the name of Szonntagh on July 28, 1981. Another electrode of this type is shown in the above-cited Butler patent.
In spite of the benefits resulting from the use of dry solid electrolytes, pH and reference electrodes that use glass body materials can only be used with test solutions at relatively low temperatures, particularly in test solutions that are strongly alkaline. This is because at pH's in excess of approximately 11 and temperatures in excess of approximately 80.degree. C., the glass used in these electrodes tends to dissolve, leading to a rapid failure of the electrode.
In response to the above-described problems, various attempts have been made to construct pH electrodes having bodies composed of materials other than glass. One family of such alternative compositions for pH electrodes comprise certain metal oxide based ceramic materials, such as those described in U.S. Pat. No. 4,264,424, issued in the name of Niedrach on Apr. 28, 1981. Among these ceramics are zirconium oxide, thorium oxide, cerium oxide and lanthanum oxide, each of which is preferably stabilized by an admixture of another metal oxide such as yttrium oxide, strontium oxide, gadolinium oxide and calcium oxide, and all of which have been collectively referred to as oxygen ion conducting ceramics.
Among the internal conductive systems described in the latter patent are: a conventional silver halide coated silver wire immersed in an aqueous halide solution, a silver halide coated silver wire immersed in an electrolyte composed of a solid or molten oxyhalide salt, and a metallic conductor immersed in an electrolyte composed of a mixture of a metal and its oxide or a mixture of metal oxides.
Of these electrolyte systems, the liquid electrolytes are unsuitable for use with high temperature test solutions because of the internal pressures incident to the vaporization of the water in the electrolyte solution at such temperatures. The oxyhalide electrolyte systems are subject to similar problems, although at higher temperatures. Silver chlorate, for example, melts at 230.degree. C. and decomposes with the release of oxygen at 270.degree. C.; silver bromate and iodate melt and decompose at even lower temperatures. The contact of the resulting high temperature oxygen with the metal within the electrode can even be expected to give rise to a condition of rapid oxidation within the electrode. The metal oxide electrolyte systems, on the other hand, are undesirable because of the instabilities associated with the fact that the oxide ion has an activity that can change, particularly in the presence of metals having a number of different oxidation states.
Prior to the present invention, the presence of the above-mentioned types of electrolytes has been thought to be necessary because of the belief that oxygen had to be present in the material that contacts the inner surface of the ceramic material in order to provide a reference for the oxygen ions in that ceramic material. One basis for this belief is set forth in U.S. Pat. No. 3,619,381 issued in the name of Fitterer on Nov. 9, 1971. Thus, the use of electrode systems including metal oxide based ceramic materials has only partially solved the problems associated with the measurement of pH at high temperatures and pressures.
Various attempts have also been made to construct reference electrodes that provide the desired ionic continuity, but which do not have a leak structure in the usual, macroscopic sense. One such reference electrode is shown in U.S. Pat. No. 4,002,547, issued in the name of Neti et al. on Jan. 11, 1977. While such reference electrodes eliminate problems with the cross-contamination of the electrolyte solution and the test solution, they cannot be used with high temperature test solutions because of the previously described internal pressure problem. Other reference electrodes, such as those described in the above-cited Szonntagh patent, have solid electrolytes with cracked glass reference bodies and are therefore subject to the above-described temperature and pH limitations.
The foregoing remarks will be understood to be applicable, in part, to combination electrode systems, i.e., electrode systems in which the pH and reference electrodes are joined into a unitary physical structure. A combination electrode based on the use of liquid electrolytes for the pH and reference electrodes is shown, for example, in U.S. Pat. No. 4,128,468 issued in the name of Bukamier on Dec. 5, 1978. Combination electrodes based on the use of solid electrolytes are shown in the above-cited Butler and Szonntagh patents.